Present day solar cell technology is regarded as too expensive to be adopted at scales sufficient to significantly reduce greenhouse gas emissions. This situation can be improved by increasing solar cell efficiency and/or by reducing cost. Nanostructured solar cells having improved efficiency have been previously proposed, but such solar cells still tend to be too costly for large-scale applications.
The fundamental technology for solar energy is the photovoltaic (PV) cell, which converts solar radiation into usable electrical energy. Over 90% of today's solar cell production is based on silicon; although silicon-based technology is relatively efficient, the severe requirements on the material crystallinity leads to costs that are currently too high to be adopted for large scale application. Hence there is a pressing need for the development of photovoltaic cells with low cost, high efficiency, and good stability.
One way to achieve low cost cells is by using thin film deposition methods to synthesize solar cell materials and devices. Demonstrated low cost technologies include both solution (e.g. chemical bath deposition, electrodeposition) and vapor (e.g. sputter deposition) methods to grow thin films. However, in thin film photovoltaic technologies, there exists a common problem with conversion efficiency due to poor materials quality: the photogenerated electrons and holes cannot travel very far before recombining at structural defects. Thus the free-carrier diffusion length is typically much shorter than the light penetration depth, so that many photogenerated carriers are lost to recombination before they can reach the device junction and produce power. FIGS. 1(a) and 1(b) show a PV structure 100 and free carrier diffusion length, respectively, that can greatly mitigate the problem by using a nano or microscale heterojunction design with interdigitated semiconductor layers 102/104, thereby obtaining a large light absorption path length 106 (optical thickness) with a short carrier diffusion path 108 to the device junction (see FIG. 1(b)). As shown in the schematic drawing of FIG. 1(a), the solar cell heterostructure design in which the p-n junctions (104/102) are oriented perpendical to the direction of solar flux 110. The interdigitated design separates the critical dimensions for light absorption and the carrier diffusion into orthogonal directions. The design allows highly efficient collection of photogenerated carriers even in poor quality materials by decoupling the length scale of carrier diffusion from that of light absorption. Recently, several investigators have begun to explore this nanostructured geometry in solar cells, mainly with semiconductor nanowires.
Any evaluation of the feasibility of using nanostructured solar cells for true large scale production must include consideration of two important issues: dark current, and the economics of nanostructuring. Studies have shown that the negative effects of short minority-carrier lifetimes, namely high dark current, will be exacerbated by the large increase in junction area that occurs with nanostructuring. Calculations show that the effect can be mitigated by proper choice of materials and by optimizing the length scale for the structure. In choosing a length scale for nanostructuring, a tradeoff must be reached in order to maximize carrier collection without excessively increasing recombination at the interfaces. It is understood that optimal efficiencies will be obtained when the size scale for the nanostructuring is approximately the same as the minority carrier diffusion length. Depending on the material, the tradeoff between carrier collection efficiency and junction area will therefore result in a desirable length scale of order 100's of nm to microns in size.
Forming such nanostructures at low cost, however, is less easily achieved within the currently available repertoire of fabrication methods. In fact, the biggest pitfall to this nanostructuring geometry is that while it may ultimately provide for higher efficiency, the difficulty of making the nanostructures using most currently available methods will drive the cost of the solar cells up, likely negating any increase in efficiency. For example, one technique used in the prior art catalyzed vapor-liquid-solid (VLS) growth of nano- (and micro-) wires. To form ordered array of nanowires, deposition of the catalyst for the VLS growth required photolithographic patterning, deposition, and etch steps, each adding significant expense. Other techniques used for forming nanowires or other nanostructures include colloidal lithography with reactive ion etching, chemical etching of bulk single crystals, and laser ablation. Each of these techniques introduces significant cost to the manufacturing process, detracting from the advantage of nanostructuring.
To overcome this limitation, a method is needed for forming nanostructured solar cells at very low cost, which does not require any of the above elaborate methods. What is needed is a method of taking advantage of natural self-organization as dictated by the phase diagram for the materials of interest.